feat(big): add Mul

storm/big/Ops.cpp

@@ -49,6 +49,19 @@ uint64_t MakeLarge(uint32_t low, uint32_t high) {
     return low + (static_cast<uint64_t>(high) << 32);
 }
 
+void Mul(BigBuffer& a, const BigBuffer& b, uint64_t c) {
+    // TODO assertion c <= SMALL_BOUND
+
+    uint64_t carry = 0;
+    uint32_t i = 0;
+    for (i = 0; carry || b.IsUsed(i); i++) {
+        carry += b[i] * c;
+        a[i] = ExtractLowPart(carry);
+    }
+
+    a.SetCount(i);
+}
+
 void ToBinaryAppend(TSGrowableArray<uint8_t>& output, const BigBuffer& buffer) {
     for (uint32_t i = 0; i < buffer.Count() * 4; i++) {
         auto byte = buffer[i / 4] >> (8 * (i & 3));

storm/big/Ops.hpp

@@ -16,6 +16,8 @@ void FromUnsigned(BigBuffer& buffer, uint32_t value);
 
 uint64_t MakeLarge(uint32_t low, uint32_t high);
 
+void Mul(BigBuffer& a, const BigBuffer& b, uint64_t c);
+
 void ToBinary(TSGrowableArray<uint8_t>& output, const BigBuffer& buffer);
 
 #endif

test/Big.cpp

@@ -54,6 +54,66 @@ TEST_CASE("ExtractLowPartSx", "[big]") {
     }
 }
 
+TEST_CASE("Mul", "[big]") {
+    SECTION("multiplies 0 and 1") {
+        BigData* a;
+        SBigNew(&a);
+
+        BigData* b;
+        SBigNew(&b);
+        SBigFromUnsigned(b, 0);
+
+        uint64_t c = 1;
+
+        Mul(a->Primary(), b->Primary(), c);
+
+        CHECK(a->Primary().Count() == 1);
+        CHECK(a->Primary()[0] == 0);
+
+        SBigDel(a);
+        SBigDel(b);
+    }
+
+    SECTION("multiplies 2 and 4") {
+        BigData* a;
+        SBigNew(&a);
+
+        BigData* b;
+        SBigNew(&b);
+        SBigFromUnsigned(b, 2);
+
+        uint64_t c = 4;
+
+        Mul(a->Primary(), b->Primary(), c);
+
+        CHECK(a->Primary().Count() == 1);
+        CHECK(a->Primary()[0] == 8);
+
+        SBigDel(a);
+        SBigDel(b);
+    }
+
+    SECTION("multiplies 0xFFFFFFFF and 0x100") {
+        BigData* a;
+        SBigNew(&a);
+
+        BigData* b;
+        SBigNew(&b);
+        SBigFromUnsigned(b, 0xFFFFFFFF);
+
+        uint64_t c = 0x100;
+
+        Mul(a->Primary(), b->Primary(), c);
+
+        CHECK(a->Primary().Count() == 2);
+        CHECK(a->Primary()[0] == 0xFFFFFF00);
+        CHECK(a->Primary()[1] == 0xFF);
+
+        SBigDel(a);
+        SBigDel(b);
+    }
+}
+
 TEST_CASE("MakeLarge", "[big]") {
     SECTION("creates uint64_t out of 0xAABBCCDD and 0x11223344") {
         uint64_t value = MakeLarge(0xAABBCCDD, 0x11223344);